package org.apache.lucene.search;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.text.Collator;
import java.util.Locale;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.search.FieldCache.DoubleParser;
import org.apache.lucene.search.FieldCache.LongParser;
import org.apache.lucene.search.FieldCache.ByteParser;
import org.apache.lucene.search.FieldCache.FloatParser;
import org.apache.lucene.search.FieldCache.IntParser;
import org.apache.lucene.search.FieldCache.ShortParser;
import org.apache.lucene.search.FieldCache.StringIndex;
import org.apache.lucene.util.Bits;

/**
 * Expert: a FieldComparator compares hits so as to determine their
 * sort order when collecting the top results with {@link
 * TopFieldCollector}.  The concrete public FieldComparator
 * classes here correspond to the SortField types.
 *
 * <p>This API is designed to achieve high performance
 * sorting, by exposing a tight interaction with {@link
 * FieldValueHitQueue} as it visits hits.  Whenever a hit is
 * competitive, it's enrolled into a virtual slot, which is
 * an int ranging from 0 to numHits-1.  The {@link
 * FieldComparator} is made aware of segment transitions
 * during searching in case any internal state it's tracking
 * needs to be recomputed during these transitions.</p>
 *
 * <p>A comparator must define these functions:</p>
 *
 * <ul>
 *
 *  <li> {@link #compare} Compare a hit at 'slot a'
 *       with hit 'slot b'.
 *
 *  <li> {@link #setBottom} This method is called by
 *       {@link FieldValueHitQueue} to notify the
 *       FieldComparator of the current weakest ("bottom")
 *       slot.  Note that this slot may not hold the weakest
 *       value according to your comparator, in cases where
 *       your comparator is not the primary one (ie, is only
 *       used to break ties from the comparators before it).
 *
 *  <li> {@link #compareBottom} Compare a new hit (docID)
 *       against the "weakest" (bottom) entry in the queue.
 *
 *  <li> {@link #copy} Installs a new hit into the
 *       priority queue.  The {@link FieldValueHitQueue}
 *       calls this method when a new hit is competitive.
 *
 *  <li> {@link #setNextReader} Invoked
 *       when the search is switching to the next segment.
 *       You may need to update internal state of the
 *       comparator, for example retrieving new values from
 *       the {@link FieldCache}.
 *
 *  <li> {@link #value} Return the sort value stored in
 *       the specified slot.  This is only called at the end
 *       of the search, in order to populate {@link
 *       FieldDoc#fields} when returning the top results.
 * </ul>
 *
 * @lucene.experimental
 */
public abstract class FieldComparator<T> {

    /**
     * Compare hit at slot1 with hit at slot2.
     * 
     * @param slot1 first slot to compare
     * @param slot2 second slot to compare
     * @return any N < 0 if slot2's value is sorted after
     * slot1, any N > 0 if the slot2's value is sorted before
     * slot1 and 0 if they are equal
     */
    public abstract int compare(int slot1, int slot2);

    /**
     * Set the bottom slot, ie the "weakest" (sorted last)
     * entry in the queue.  When {@link #compareBottom} is
     * called, you should compare against this slot.  This
     * will always be called before {@link #compareBottom}.
     * 
     * @param slot the currently weakest (sorted last) slot in the queue
     */
    public abstract void setBottom(final int slot);

    /**
     * Compare the bottom of the queue with doc.  This will
     * only invoked after setBottom has been called.  This
     * should return the same result as {@link
     * #compare(int,int)}} as if bottom were slot1 and the new
     * document were slot 2.
     *    
     * <p>For a search that hits many results, this method
     * will be the hotspot (invoked by far the most
     * frequently).</p>
     * 
     * @param doc that was hit
     * @return any N < 0 if the doc's value is sorted after
     * the bottom entry (not competitive), any N > 0 if the
     * doc's value is sorted before the bottom entry and 0 if
     * they are equal.
     */
    public abstract int compareBottom(int doc) throws IOException;

    /**
     * This method is called when a new hit is competitive.
     * You should copy any state associated with this document
     * that will be required for future comparisons, into the
     * specified slot.
     * 
     * @param slot which slot to copy the hit to
     * @param doc docID relative to current reader
     */
    public abstract void copy(int slot, int doc) throws IOException;

    /**
     * Set a new Reader. All doc correspond to the current Reader.
     * 
     * @param reader current reader
     * @param docBase docBase of this reader 
     * @throws IOException
     * @throws IOException
     */
    public abstract void setNextReader(IndexReader reader, int docBase) throws IOException;

    /** Sets the Scorer to use in case a document's score is
     *  needed.
     * 
     * @param scorer Scorer instance that you should use to
     * obtain the current hit's score, if necessary. */
    public void setScorer(Scorer scorer) {
        // Empty implementation since most comparators don't need the score. This
        // can be overridden by those that need it.
    }

    /**
     * Return the actual value in the slot.
     *
     * @param slot the value
     * @return value in this slot
     */
    public abstract T value(int slot);

    /** Returns -1 if first is less than second.  Default
     *  impl to assume the type implements Comparable and
     *  invoke .compareTo; be sure to override this method if
     *  your FieldComparator's type isn't a Comparable or
     *  if your values may sometimes be null */
    @SuppressWarnings("unchecked")
    public int compareValues(T first, T second) {
        if (first == null) {
            if (second == null) {
                return 0;
            } else {
                return -1;
            }
        } else if (second == null) {
            return 1;
        } else {
            return ((Comparable<T>) first).compareTo(second);
        }
    }

    public static abstract class NumericComparator<T extends Number> extends FieldComparator<T> {
        protected final T missingValue;
        protected final String field;
        protected Bits docsWithField;

        public NumericComparator(String field, T missingValue) {
            this.field = field;
            this.missingValue = missingValue;
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            if (missingValue != null) {
                docsWithField = FieldCache.DEFAULT.getDocsWithField(reader, field);
                // optimization to remove unneeded checks on the bit interface:
                if (docsWithField instanceof Bits.MatchAllBits) {
                    docsWithField = null;
                }
            } else {
                docsWithField = null;
            }
        }
    }

    /** Parses field's values as byte (using {@link
     *  FieldCache#getBytes} and sorts by ascending value */
    public static final class ByteComparator extends NumericComparator<Byte> {
        private final byte[] values;
        private final ByteParser parser;
        private byte[] currentReaderValues;
        private byte bottom;

        ByteComparator(int numHits, String field, FieldCache.Parser parser, Byte missingValue) {
            super(field, missingValue);
            values = new byte[numHits];
            this.parser = (ByteParser) parser;
        }

        @Override
        public int compare(int slot1, int slot2) {
            return values[slot1] - values[slot2];
        }

        @Override
        public int compareBottom(int doc) {
            byte v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            return bottom - v2;
        }

        @Override
        public void copy(int slot, int doc) {
            byte v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            values[slot] = v2;
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            // NOTE: must do this before calling super otherwise
            // we compute the docsWithField Bits twice!
            currentReaderValues = FieldCache.DEFAULT.getBytes(reader, field, parser, missingValue != null);
            super.setNextReader(reader, docBase);
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = values[bottom];
        }

        @Override
        public Byte value(int slot) {
            return Byte.valueOf(values[slot]);
        }
    }

    /** Sorts by ascending docID */
    public static final class DocComparator extends FieldComparator<Integer> {
        private final int[] docIDs;
        private int docBase;
        private int bottom;

        DocComparator(int numHits) {
            docIDs = new int[numHits];
        }

        @Override
        public int compare(int slot1, int slot2) {
            // No overflow risk because docIDs are non-negative
            return docIDs[slot1] - docIDs[slot2];
        }

        @Override
        public int compareBottom(int doc) {
            // No overflow risk because docIDs are non-negative
            return bottom - (docBase + doc);
        }

        @Override
        public void copy(int slot, int doc) {
            docIDs[slot] = docBase + doc;
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) {
            // TODO: can we "map" our docIDs to the current
            // reader? saves having to then subtract on every
            // compare call
            this.docBase = docBase;
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = docIDs[bottom];
        }

        @Override
        public Integer value(int slot) {
            return Integer.valueOf(docIDs[slot]);
        }
    }

    /** Parses field's values as double (using {@link
     *  FieldCache#getDoubles} and sorts by ascending value */
    public static final class DoubleComparator extends NumericComparator<Double> {
        private final double[] values;
        private final DoubleParser parser;
        private double[] currentReaderValues;
        private double bottom;

        DoubleComparator(int numHits, String field, FieldCache.Parser parser, Double missingValue) {
            super(field, missingValue);
            values = new double[numHits];
            this.parser = (DoubleParser) parser;
        }

        @Override
        public int compare(int slot1, int slot2) {
            final double v1 = values[slot1];
            final double v2 = values[slot2];
            if (v1 > v2) {
                return 1;
            } else if (v1 < v2) {
                return -1;
            } else {
                return 0;
            }
        }

        @Override
        public int compareBottom(int doc) {
            double v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            if (bottom > v2) {
                return 1;
            } else if (bottom < v2) {
                return -1;
            } else {
                return 0;
            }
        }

        @Override
        public void copy(int slot, int doc) {
            double v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            values[slot] = v2;
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            // NOTE: must do this before calling super otherwise
            // we compute the docsWithField Bits twice!
            currentReaderValues = FieldCache.DEFAULT.getDoubles(reader, field, parser, missingValue != null);
            super.setNextReader(reader, docBase);
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = values[bottom];
        }

        @Override
        public Double value(int slot) {
            return Double.valueOf(values[slot]);
        }
    }

    /** Parses field's values as float (using {@link
     *  FieldCache#getFloats} and sorts by ascending value */
    public static final class FloatComparator extends NumericComparator<Float> {
        private final float[] values;
        private final FloatParser parser;
        private float[] currentReaderValues;
        private float bottom;

        FloatComparator(int numHits, String field, FieldCache.Parser parser, Float missingValue) {
            super(field, missingValue);
            values = new float[numHits];
            this.parser = (FloatParser) parser;
        }

        @Override
        public int compare(int slot1, int slot2) {
            // TODO: are there sneaky non-branch ways to compute
            // sign of float?
            final float v1 = values[slot1];
            final float v2 = values[slot2];
            if (v1 > v2) {
                return 1;
            } else if (v1 < v2) {
                return -1;
            } else {
                return 0;
            }
        }

        @Override
        public int compareBottom(int doc) {
            // TODO: are there sneaky non-branch ways to compute
            // sign of float?
            float v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            if (bottom > v2) {
                return 1;
            } else if (bottom < v2) {
                return -1;
            } else {
                return 0;
            }
        }

        @Override
        public void copy(int slot, int doc) {
            float v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            values[slot] = v2;
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            // NOTE: must do this before calling super otherwise
            // we compute the docsWithField Bits twice!
            currentReaderValues = FieldCache.DEFAULT.getFloats(reader, field, parser, missingValue != null);
            super.setNextReader(reader, docBase);
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = values[bottom];
        }

        @Override
        public Float value(int slot) {
            return Float.valueOf(values[slot]);
        }
    }

    /** Parses field's values as int (using {@link
     *  FieldCache#getInts} and sorts by ascending value */
    public static final class IntComparator extends NumericComparator<Integer> {
        private final int[] values;
        private final IntParser parser;
        private int[] currentReaderValues;
        private int bottom; // Value of bottom of queue

        IntComparator(int numHits, String field, FieldCache.Parser parser, Integer missingValue) {
            super(field, missingValue);
            values = new int[numHits];
            this.parser = (IntParser) parser;
        }

        @Override
        public int compare(int slot1, int slot2) {
            // TODO: there are sneaky non-branch ways to compute
            // -1/+1/0 sign
            // Cannot return values[slot1] - values[slot2] because that
            // may overflow
            final int v1 = values[slot1];
            final int v2 = values[slot2];
            if (v1 > v2) {
                return 1;
            } else if (v1 < v2) {
                return -1;
            } else {
                return 0;
            }
        }

        @Override
        public int compareBottom(int doc) {
            // TODO: there are sneaky non-branch ways to compute
            // -1/+1/0 sign
            // Cannot return bottom - values[slot2] because that
            // may overflow
            int v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            if (bottom > v2) {
                return 1;
            } else if (bottom < v2) {
                return -1;
            } else {
                return 0;
            }
        }

        @Override
        public void copy(int slot, int doc) {
            int v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            values[slot] = v2;
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            // NOTE: must do this before calling super otherwise
            // we compute the docsWithField Bits twice!
            currentReaderValues = FieldCache.DEFAULT.getInts(reader, field, parser, missingValue != null);
            super.setNextReader(reader, docBase);
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = values[bottom];
        }

        @Override
        public Integer value(int slot) {
            return Integer.valueOf(values[slot]);
        }
    }

    /** Parses field's values as long (using {@link
     *  FieldCache#getLongs} and sorts by ascending value */
    public static final class LongComparator extends NumericComparator<Long> {
        private final long[] values;
        private final LongParser parser;
        private long[] currentReaderValues;
        private long bottom;

        LongComparator(int numHits, String field, FieldCache.Parser parser, Long missingValue) {
            super(field, missingValue);
            values = new long[numHits];
            this.parser = (LongParser) parser;
        }

        @Override
        public int compare(int slot1, int slot2) {
            // TODO: there are sneaky non-branch ways to compute
            // -1/+1/0 sign
            final long v1 = values[slot1];
            final long v2 = values[slot2];
            if (v1 > v2) {
                return 1;
            } else if (v1 < v2) {
                return -1;
            } else {
                return 0;
            }
        }

        @Override
        public int compareBottom(int doc) {
            // TODO: there are sneaky non-branch ways to compute
            // -1/+1/0 sign
            long v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            if (bottom > v2) {
                return 1;
            } else if (bottom < v2) {
                return -1;
            } else {
                return 0;
            }
        }

        @Override
        public void copy(int slot, int doc) {
            long v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            values[slot] = v2;
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            // NOTE: must do this before calling super otherwise
            // we compute the docsWithField Bits twice!
            currentReaderValues = FieldCache.DEFAULT.getLongs(reader, field, parser, missingValue != null);
            super.setNextReader(reader, docBase);
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = values[bottom];
        }

        @Override
        public Long value(int slot) {
            return Long.valueOf(values[slot]);
        }
    }

    /** Sorts by descending relevance.  NOTE: if you are
     *  sorting only by descending relevance and then
     *  secondarily by ascending docID, performance is faster
     *  using {@link TopScoreDocCollector} directly (which {@link
     *  IndexSearcher#search} uses when no {@link Sort} is
     *  specified). */
    public static final class RelevanceComparator extends FieldComparator<Float> {
        private final float[] scores;
        private float bottom;
        private Scorer scorer;

        RelevanceComparator(int numHits) {
            scores = new float[numHits];
        }

        @Override
        public int compare(int slot1, int slot2) {
            final float score1 = scores[slot1];
            final float score2 = scores[slot2];
            return score1 > score2 ? -1 : (score1 < score2 ? 1 : 0);
        }

        @Override
        public int compareBottom(int doc) throws IOException {
            float score = scorer.score();
            return bottom > score ? -1 : (bottom < score ? 1 : 0);
        }

        @Override
        public void copy(int slot, int doc) throws IOException {
            scores[slot] = scorer.score();
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) {
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = scores[bottom];
        }

        @Override
        public void setScorer(Scorer scorer) {
            // wrap with a ScoreCachingWrappingScorer so that successive calls to
            // score() will not incur score computation over and
            // over again.
            if (!(scorer instanceof ScoreCachingWrappingScorer)) {
                this.scorer = new ScoreCachingWrappingScorer(scorer);
            } else {
                this.scorer = scorer;
            }
        }

        @Override
        public Float value(int slot) {
            return Float.valueOf(scores[slot]);
        }

        // Override because we sort reverse of natural Float order:
        @Override
        public int compareValues(Float first, Float second) {
            // Reversed intentionally because relevance by default
            // sorts descending:
            return second.compareTo(first);
        }
    }

    /** Parses field's values as short (using {@link
     *  FieldCache#getShorts} and sorts by ascending value */
    public static final class ShortComparator extends NumericComparator<Short> {
        private final short[] values;
        private final ShortParser parser;
        private short[] currentReaderValues;
        private short bottom;

        ShortComparator(int numHits, String field, FieldCache.Parser parser, Short missingValue) {
            super(field, missingValue);
            values = new short[numHits];
            this.parser = (ShortParser) parser;
        }

        @Override
        public int compare(int slot1, int slot2) {
            return values[slot1] - values[slot2];
        }

        @Override
        public int compareBottom(int doc) {
            short v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            return bottom - v2;
        }

        @Override
        public void copy(int slot, int doc) {
            short v2 = currentReaderValues[doc];
            // Test for v2 == 0 to save Bits.get method call for
            // the common case (doc has value and value is non-zero):
            if (docsWithField != null && v2 == 0 && !docsWithField.get(doc)) {
                v2 = missingValue;
            }
            values[slot] = v2;
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            // NOTE: must do this before calling super otherwise
            // we compute the docsWithField Bits twice!
            currentReaderValues = FieldCache.DEFAULT.getShorts(reader, field, parser, missingValue != null);
            super.setNextReader(reader, docBase);
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = values[bottom];
        }

        @Override
        public Short value(int slot) {
            return Short.valueOf(values[slot]);
        }
    }

    /** Sorts by a field's value using the Collator for a
     *  given Locale.*/
    public static final class StringComparatorLocale extends FieldComparator<String> {

        private final String[] values;
        private String[] currentReaderValues;
        private final String field;
        final Collator collator;
        private String bottom;

        StringComparatorLocale(int numHits, String field, Locale locale) {
            values = new String[numHits];
            this.field = field;
            collator = Collator.getInstance(locale);
        }

        @Override
        public int compare(int slot1, int slot2) {
            final String val1 = values[slot1];
            final String val2 = values[slot2];
            if (val1 == null) {
                if (val2 == null) {
                    return 0;
                }
                return -1;
            } else if (val2 == null) {
                return 1;
            }
            return collator.compare(val1, val2);
        }

        @Override
        public int compareBottom(int doc) {
            final String val2 = currentReaderValues[doc];
            if (bottom == null) {
                if (val2 == null) {
                    return 0;
                }
                return -1;
            } else if (val2 == null) {
                return 1;
            }
            return collator.compare(bottom, val2);
        }

        @Override
        public void copy(int slot, int doc) {
            values[slot] = currentReaderValues[doc];
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            currentReaderValues = FieldCache.DEFAULT.getStrings(reader, field);
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = values[bottom];
        }

        @Override
        public String value(int slot) {
            return values[slot];
        }

        @Override
        public int compareValues(String val1, String val2) {
            if (val1 == null) {
                if (val2 == null) {
                    return 0;
                }
                return -1;
            } else if (val2 == null) {
                return 1;
            }
            return collator.compare(val1, val2);
        }
    }

    /** Sorts by field's natural String sort order, using
     *  ordinals.  This is functionally equivalent to {@link
     *  StringValComparator}, but it first resolves the string
     *  to their relative ordinal positions (using the index
     *  returned by {@link FieldCache#getStringIndex}), and
     *  does most comparisons using the ordinals.  For medium
     *  to large results, this comparator will be much faster
     *  than {@link StringValComparator}.  For very small
     *  result sets it may be slower. */
    public static final class StringOrdValComparator extends FieldComparator<String> {

        private final int[] ords;
        private final String[] values;
        private final int[] readerGen;

        private int currentReaderGen = -1;
        private String[] lookup;
        private int[] order;
        private final String field;

        private int bottomSlot = -1;
        private int bottomOrd;
        private boolean bottomSameReader;
        private String bottomValue;

        public StringOrdValComparator(int numHits, String field, int sortPos, boolean reversed) {
            ords = new int[numHits];
            values = new String[numHits];
            readerGen = new int[numHits];
            this.field = field;
        }

        @Override
        public int compare(int slot1, int slot2) {
            if (readerGen[slot1] == readerGen[slot2]) {
                return ords[slot1] - ords[slot2];
            }

            final String val1 = values[slot1];
            final String val2 = values[slot2];
            if (val1 == null) {
                if (val2 == null) {
                    return 0;
                }
                return -1;
            } else if (val2 == null) {
                return 1;
            }
            return val1.compareTo(val2);
        }

        @Override
        public int compareBottom(int doc) {
            assert bottomSlot != -1;
            if (bottomSameReader) {
                // ord is precisely comparable, even in the equal case
                return bottomOrd - this.order[doc];
            } else {
                // ord is only approx comparable: if they are not
                // equal, we can use that; if they are equal, we
                // must fallback to compare by value
                final int order = this.order[doc];
                final int cmp = bottomOrd - order;
                if (cmp != 0) {
                    return cmp;
                }

                final String val2 = lookup[order];
                if (bottomValue == null) {
                    if (val2 == null) {
                        return 0;
                    }
                    // bottom wins
                    return -1;
                } else if (val2 == null) {
                    // doc wins
                    return 1;
                }
                return bottomValue.compareTo(val2);
            }
        }

        @Override
        public void copy(int slot, int doc) {
            final int ord = order[doc];
            ords[slot] = ord;
            assert ord >= 0;
            values[slot] = lookup[ord];
            readerGen[slot] = currentReaderGen;
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            StringIndex currentReaderValues = FieldCache.DEFAULT.getStringIndex(reader, field);
            currentReaderGen++;
            order = currentReaderValues.order;
            lookup = currentReaderValues.lookup;
            assert lookup.length > 0;
            if (bottomSlot != -1) {
                setBottom(bottomSlot);
            }
        }

        @Override
        public void setBottom(final int bottom) {
            bottomSlot = bottom;

            bottomValue = values[bottomSlot];
            if (currentReaderGen == readerGen[bottomSlot]) {
                bottomOrd = ords[bottomSlot];
                bottomSameReader = true;
            } else {
                if (bottomValue == null) {
                    ords[bottomSlot] = 0;
                    bottomOrd = 0;
                    bottomSameReader = true;
                    readerGen[bottomSlot] = currentReaderGen;
                } else {
                    final int index = binarySearch(lookup, bottomValue);
                    if (index < 0) {
                        bottomOrd = -index - 2;
                        bottomSameReader = false;
                    } else {
                        bottomOrd = index;
                        // exact value match
                        bottomSameReader = true;
                        readerGen[bottomSlot] = currentReaderGen;
                        ords[bottomSlot] = bottomOrd;
                    }
                }
            }
        }

        @Override
        public String value(int slot) {
            return values[slot];
        }

        @Override
        public int compareValues(String val1, String val2) {
            if (val1 == null) {
                if (val2 == null) {
                    return 0;
                }
                return -1;
            } else if (val2 == null) {
                return 1;
            }
            return val1.compareTo(val2);
        }

        public String[] getValues() {
            return values;
        }

        public int getBottomSlot() {
            return bottomSlot;
        }

        public String getField() {
            return field;
        }
    }

    /** Sorts by field's natural String sort order.  All
     *  comparisons are done using String.compareTo, which is
     *  slow for medium to large result sets but possibly
     *  very fast for very small results sets. */
    public static final class StringValComparator extends FieldComparator<String> {

        private String[] values;
        private String[] currentReaderValues;
        private final String field;
        private String bottom;

        StringValComparator(int numHits, String field) {
            values = new String[numHits];
            this.field = field;
        }

        @Override
        public int compare(int slot1, int slot2) {
            final String val1 = values[slot1];
            final String val2 = values[slot2];
            if (val1 == null) {
                if (val2 == null) {
                    return 0;
                }
                return -1;
            } else if (val2 == null) {
                return 1;
            }

            return val1.compareTo(val2);
        }

        @Override
        public int compareBottom(int doc) {
            final String val2 = currentReaderValues[doc];
            if (bottom == null) {
                if (val2 == null) {
                    return 0;
                }
                return -1;
            } else if (val2 == null) {
                return 1;
            }
            return bottom.compareTo(val2);
        }

        @Override
        public void copy(int slot, int doc) {
            values[slot] = currentReaderValues[doc];
        }

        @Override
        public void setNextReader(IndexReader reader, int docBase) throws IOException {
            currentReaderValues = FieldCache.DEFAULT.getStrings(reader, field);
        }

        @Override
        public void setBottom(final int bottom) {
            this.bottom = values[bottom];
        }

        @Override
        public String value(int slot) {
            return values[slot];
        }

        @Override
        public int compareValues(String val1, String val2) {
            if (val1 == null) {
                if (val2 == null) {
                    return 0;
                }
                return -1;
            } else if (val2 == null) {
                return 1;
            } else {
                return val1.compareTo(val2);
            }
        }
    }

    final protected static int binarySearch(String[] a, String key) {
        return binarySearch(a, key, 0, a.length - 1);
    }

    final protected static int binarySearch(String[] a, String key, int low, int high) {

        while (low <= high) {
            int mid = (low + high) >>> 1;
            String midVal = a[mid];
            int cmp;
            if (midVal != null) {
                cmp = midVal.compareTo(key);
            } else {
                cmp = -1;
            }

            if (cmp < 0)
                low = mid + 1;
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid;
        }
        return -(low + 1);
    }
}
